Add address to string conversion

[pcaversaccio]

This PR adds a new function to the Strings library that converts an address to its ASCII string hexadecimal representation. The main motivation is the reason that I've encountered already multiple projects that use a sophisticated tokenURI strategy, that concatenates multiple attributes - one of which is an address - into a string. An example:

return string(abi.encodePacked(_baseTokenURI, toString(creator), "-", tokenId.toString()));

PR Checklist

• Tests
• Documentation => should be automatically added to Strings
• Changelog entry

[frangio]

Thank you @pcaversaccio! I agree I can imagine this will be very handy for on-chain NFT metadata.

[Amxx]

Maybe we say that the produced address is not checksummed.

[pcaversaccio]

@Amxx good point - I added the comment! Somehow the CI actions don't work on this PR?

[frangio]

@pcaversaccio It may have been a GitHub problem.

[frangio]

Thanks @pcaversaccio! I made some small changes to the tests.